There is a continuing need for high temperature NOx sensors for controlling combustion environments to meet government regulations and minimize negative effects of NOx on ecosystems and health. The two main types of electrochemical sensors that have been tested for NOx are semiconductor sensors and potentiometric sensors. One of the main drawbacks of these sensors that has hindered their development is the lack of selectivity between the two main NOx components of interest, NO and NO2.
In combustion environments NO is often the dominant NOx species with NO2 being present to a lesser amount. However, the majority of sensors cannot distinguish between the two species giving a signal response to both NO and NO2. Most solid-state sensors are dedicated to detecting NO only as NO is the major component of NOx at high temperatures. However, depending on the temperature and oxygen content NO2 can also be present and sensors that measure total NOx (NO+NO2) are required.
With electrochemical sensors, NO2 generally tends to get reduced and NO tends to be oxidized to generate opposite electrical signals. CO is a major component in a typical combustion exhaust and tends to readily oxidize to CO2. As a result, the electrical signal generated by the oxidation of CO can obscure the NOx signal. Interference due to changes in O2 concentration is also considered to be a major issue.
There have been attempts to solve this problem by using multi-chamber designs that measure total NOx and minimize interference due to CO and O2. However, these multi-chamber designs are very complicated and difficult to manufacture.